Add a "How to Add a New Backend" guide (#87)

Author: Gil Mizrahi

README.md

@@ -3,7 +3,16 @@ Try PureScript
 
 [Try PureScript](https://try.purescript.org) is an online PureScript code editor for quickly experimenting with PureScript code snippets and ideas.
 
-### Features:
+## How to Add a New Backend
+
+- Create a `bower.json`/`psc-package.json` file with all the dependencies you want to be available for the library backend and optionally an extra `Try` module if you want. [Example PR](https://github.com/purescript/trypurescript/pull/84/files)
+- Write an example in a github gist and name it `Main.purs`. [Example Gist](https://gist.github.com/ff1e87f0872d2d891e77d209d8f7706d)
+- add a backend button and a backend option in `index.html` and `index.js` respectively. [Example PR](https://github.com/purescript/trypurescript/pull/85/files)
+    - Note to link the gist from the previous bullet in the `mainGist` field in `index.js`
+- Ask phil to update the `core` main gist to link to the new backend
+
+
+## Features:
 
 - Writing code using the [Ace Editor](http://ace.c9.io)
 - Automatic compilation
@@ -16,19 +25,19 @@ Try PureScript
     - (_Note: These Gists are not associated with your GitHub account and are visible to anyone with a link to them_)
 
 
-### Control Features via the Query String
+## Control Features via the Query String
 
 Most of these features can be controlled not only from the toolbar, but also using the [query parameters](https://en.wikipedia.org/wiki/Query_string):
 
 - **Load From Gist**: Load PureScript code from Gist id using the `gist` parameter
-    - Example: `gist=37c3c97f47a43f20c548` will load the code from this Gist
+    - Example: `gist=37c3c97f47a43f20c548` will load the code from this Gist if the file was named `Main.purs`
 
 - **View Mode**: Control the view mode using the `view` parameter
     - Options are: `code`, `output`, `both`
     - Example: `view=output` will only display the output
 
 - **Backend**: Control which backend will compile your code using the `backend` parameter
-    - Options are: `core`, `thermite`, `slides`, `flare`, `mathbox`
+    - Options are: `core`, `thermite`, `slides`, `flare`, `mathbox`, `behavior`
     - Example: `backend=thermite` will use the thermite backend
 
 - **Auto Compile**: Automatic compilation can be turned off using the `compile` parameter
@@ -44,8 +53,10 @@ Most of these features can be controlled not only from the toolbar, but also usi
     - Example: `session=9162f098-070f-4053-60ea-eba47021450d` (Note: will probably not work for you)
     - When used with the `gist` query parameter the code will be loaded from the Gist and not the session
 
-### Packages
+## Packages
 
 - The packages set and compiler version for Try-PureScript can be viewed [here](https://github.com/purescript/trypurescript/tree/master/staging/core/psc-package.json).
 
 - The packages set and compiler version for Try-Thermite can be viewed [here](https://github.com/paf31/try-thermite/blob/gh-pages/staging/psc-package.json).
+
+